1. Field of the Invention
The present invention relates to an engine having balance shafts disposed below a cylinder block, and more particularly to a supporting structure for the balance shafts of the engine.
2. Description of the Related Art
There have been existing heretofore engines having a balancer below the cylinder block (for instance, refer to the Japanese Patent Unexamined Publication (Kokai) No. Sho. 63-106443). In an engine of this type, the balancer comprises a pair of balance shafts supported on a casing disposed in an oil pan below the cylinder block. The balance shafts are intermeshed with each other, and the driving force of a crank shafts is transmitted therefrom to one of the balance shafts via a chain or the like, whereby the two balance shafts are rotated at twice the crankshaft revolution speed in opposite directions.
On the other hand, in the above construction, an oil pump is also disposed below the engine closer to the balancer with a view to making it smaller. To make this happen, it is desirable that the balancer casing and the oil pump body are made integral with each other.
In general, the aforementioned balancer shafts are supported at a plurality positions. With a large-diameter journal portion of the balance shaft that is rotatably supported by a bearing thereof, when rotating, the speed is increased at which the journal portion rotates while in sliding contact with the bearing, and since this causes the bearing and journal portion to get worn largely, it is preferable to make the diameter of the journal portion as small as possible to such an extent that the strength thereof can be secured.
As disclosed in the official gazette of Japanese Patent Unexamined Publication (Kokai) No. Hei. 9-210135, however, in a construction in which balance shafts are inserted from an axial direction, it is at least needed that a journal portion is made to have the largest diameter.
To cope with this, it is considered that a casing as described above and bearings for the balance shafts are divided in two halves in vertical directions, but when the entirety of the balancer is constructed to be so divided, an oil pump body cannot be molded integrally with the casing and therefore it has to be retrofitted, this leading to a drawback of increasing the number of components and assembling man hours.
On the other hand, it is possible that two casing halves including an oil pump body and an oil pump cap are subassembled and that thereafter mounting surfaces to the cylinder block of such a sub-assembly are processed, but this deteriorates the workability. Therefore, it is desirable that a mounting surface of each component to the cylinder block be processed, but this tends to cause processing errors and in a worse case, the balance shafts incline and this causes partial wear.